The proposed studies will examine two major aspects of influenza virus neutralization: 1) the capacity of antiviral monoclonal antibodies to interfere with virus-cell interactions leading to virus neutralization, and 2) the factors which determine the neutralizing potency of anti-hemagglutinin (HA) monoclonal antibodies. Biochemical and morphological techniques will be used to study the capacity of antiviral antibodies to interfere with the attachment of virus to cells, the endocytosis of virus particles and the fusion of viral and cellular membranes. In addition the stoichiometry of virus neutralization will be studied by determining the average number of radiolabeled antibodies bound to virus particles in virus preparations showing a 50 percent decrease in infectivity. Studies will also be initiated to examine why anti-HA monoclonal antibodies vary enormously in their capacity to neutralize virus.